1. Field of Invention
This invention relates in general to the field of slurry pumping systems and, in particular, to a novel method to increase the solids to fluid ratio of the slurries pumped.
2. Description of Prior Art
Slurry pumping systems are used throughout industry to transport bulk solids materials. Slurry is defined as a mixture of particulate solid material and some liquid that is used as the carrier (transport) fluid. Materials transported by slurry methods include sand, gravel, coal, ore, earthy materials, sewerage sludge, vegetables, and other materials. Within the slurry pumping industry, the term "slurry density" refers to the ratio of solids weight to total slurry weight. That term is used within this patent application.
Slurry density is one of the primary determinants of the energy efficiency of the slurry pumping system. For each system, there is an ideal slurry density which is a complex function of pipeline diameter, solid particle density, solid particle size distribution, carrier fluid density, carrier fluid viscosity, and other factors. Pumping systems that operate with below optimum slurry density waste energy by pumping excessive amounts of carrier fluid.
Many slurry systems are operated below the optimum slurry density. This situation frequently occurs because the intake pump cavitates when "suction head required" for dense slurry exceeds the "suction head available" from the intake pump. Low intake slurry density also frequently occurs in sluice mining operations where mining water requirements are greater than optimum transport water requirements.
Several gravitational settling devices have been proposed in which the slurry is directed to a large settling basin where the solids settle to the bottom and are withdrawn under controlled environments. Refer to U.S. Pat. No. 3,870,373 "Underground Coal Slurry Concentrating Sump", U.S. Pat. No. 3,924,895 "Method and Apparatus for Hydraulic Transportation of Coal", U.S. Pat. No. 3,942,841 "Slurry Handling System", and U.S. Pat. No. 3,981,541 "Shallow Underground Coal Slurry Concentration Sump". These methods are not preferred for two reasons. Firstly, the designs make no effort to minimize the size of the settling basins. In cases where a large portion of the solids is comprised of very small particles, the described settling basins would be very large. In these cases, the described settling basins could not be portable and therefore could not be kept adjacent to the mining activity. Secondly, all of the described basins are open to atmosphere so that all of the incoming slurry pressure energy is lost when it is delivered to the settling basin. Therefore, it is seen that these simple settling basins are not satisfactory when a mobile, energy efficient means to increase slurry density is required.
U.S. Pat. No. 4,415,296 "Materials Transfer System and Methodology" overcomes the energy efficiency objection described in the previous paragraph. The "separating device" is maintained under the pressure of the "transfer device" and thus does not lose this energy. However, U.S. Pat. No. 4,415,296 provides no criteria to teach design of the separating device beyond conventional equipment-screens, hydrocyclones, and settling tanks. These equipment units would be too large for easy mobility. Hydrocyclone equipment and screen equipment would be very large because all of the flow discharged by the "transfer device" is treated and because most ore slurries will contain a wide range of particle sizes that will necessitate multiple stages of processing. Conventional settling tanks would have to be large when treating the full slurry volume. Furthermore, conventional settling tanks are constructed as one large diameter tank that would be very heavy in order to be strong enough to withstand the pumping system operating pressure. The device taught by U.S. Pat. No. 4,415,296 does not provide protection against plugging the downstream pipeline due to inadequate slurry velocity. When the intake pump discharges an interval of very low density slurry the device of U.S. Pat. No. 4,415,296 could divert too much slurry away from the downstream pipeline. In this case, the controller should limit the amount of slurry diverted in order to maintain a pre-set minimum slurry velocity.